I was thinking Mr Freeze, he's an interesting character when developed correctly, but would the stigma of Batman and Robin stop a film with freeze as the lead villain being successful? Cinema-goers tend to have long memories.

WB will want a succesful film, so i would assume they would want the Joker, as he's the most famous member of the Rogue's gallery. If they do Joker again, this time they have to put Harley Quinn in it, just to change it up a bit, and try to distract people from comparing this Joker to Ledger and Nicholson.